Shoutbox

If I remember correctly, the MSN Beta versions had debugging features in them, which is most likely what is happening here. This makes it easier to find bugs, but makes memory usage get pretty high.
Hopefully in the release version they'll take these out, will probably cut a bit of memory overhead off it.

On the other hand, new features in this build from the last one I had (566 I think):

• * New Sign in Sounds - Don't care for it much myself. When attempting to change these, there is now a built in options form for them, you don't have to open windows sounds.
  * New icons - makes it easier to distinguist online / offline contacts, and who is blocked.
  * The title bar of the contact list has changed colour.
  * When a contact sends you more than one message, it will no longer show their nick twice. Can be turned back on under the "Messages" tab.
  * A new option in "General": Show shadows and animations. Haven't seen it do anything yet.
  * You can now disable Custom Emoticons without disabling the built-in ones.
  * An option under "File Transfer" to install "Windows Live Safety Scanner". Sounds fancy.

There's probably more, but I haven't found a full list yet.